1. Field of the Invention
The present invention relates generally to a magnetic stirring apparatus, and more particularly, to a magnetic stirring apparatus utilizing sine wave amplitude currents in a circular magnetic field generator to allow very low rotational speeds of a stirring bar, e.g., 1 rpm.
2. Description of the Related Art
A magnetic stirrer is a device which spins a small permanent bar magnet (usually Teflon coated and called a stir bar) placed inside a container or vessel of liquid, causing the liquid to be stirred.
Earlier prior art magnetic stirrers accomplished this by attaching a second permanent bar magnet to the top of a shaft of an electric motor, with the second magnet perpendicular to the shaft, and the axis of the motor shaft attached to the center of the magnet. When the second magnet on the motor shaft is placed in close proximity to the magnet in the vessel (usually by placing the motor in a non-magnetic housing with the second magnet at the top very close to the housing, and placing the vessel of liquid on top of the housing), the two magnets couple together, and the magnet in the vessel, i.e., stir bar, spins at the same rate as the second magnet attached to the motor. However, these stirrers tended to stall at low speeds due to friction and resistance in the bearings and wear out after many hours of use.
Later magnetic stirrers eliminated the motor and its associated moving parts by generating a rotating magnetic field beneath the vessel by providing at least one pair of parallelly disposed magnet coils supplied with respective phase shifted ac currents. The phase shifted ac currents resulted in alternating magnetic polarities of the coils that subjected the poles of the stir bar to alternating attracting and repelling forces, causing the stir bar to rotate about it axis.
Others have improved upon the coil method mentioned above by employing four or more coils of wire (usually wrapped around a ferrous post) arranged around a center point with tabs attached to the ferrous posts and pointing to the center point. Electric currents are applied to the coils to create magnetic fields above the coils and in the tabs to couple and rotate the stir bar. One such stirrer is disclosed in U.S. Pat. No. 4,199,265, entitled “MOTORLESS MAGNETICALLY COUPLED STIRRER” issued to Sanderson et al. on Apr. 22, 1980. One coil is energized to create a north magnetic pole in the tab connected to it, while the opposite coil is energized to create a south pole in the tab. The stir bar then aligns it's south pole over the north energized coil and it's north pole above the south energized coil. The current is then turned off, and (in the case of 4 coils) the other two coils are energized in this north/south manner. The stir bar then swings 90 degrees to align with the next set of coils. Current is then applied to the original set of coils, but reversed in direction, and the stir bar rotates 90 degrees more. This switching of coil current is continued, to keep the stir bar rotating. Effectively, a series of pulses is being sent to the magnetic coils for sequentially stepping a magnetic field in a rotary motion about the center point of the tabs. As long as the current is switched fast enough (approx 80-100 rpm), the stir bar moves smoothly, and the liquid is effectively stirred.
Slow speed magnetic stirring (1-80 rpm), is more difficult, and is usually achieved only by the above-mentioned method of attaching a magnet to a slow speed motor. Others have attempted to use the coil method, however, the magnetic attraction of the stir bar to the metal tabs causes very choppy motion at slow speeds, which is unacceptable, especially for biological applications.